I have a few questions, we can choose 3 species for our MC but Nerissa is a human so it is safe to say King Fenris is also human? Then our non-human MC is half-tilaari or half-kitalphan right? The Queens( Stellar, Lunar and Solar ) all existed and are married to King Fenris even if we do not choose their respective species as the MCs? Are there more Queens besides the three? Both human MC and Nerissa are children of Queen Lucrezia, does she have any other children? The other two Queens have any other children besides the MC?

Yes, King Fenris is human. So also yes to MC being either half-Tilaari or half-Kitalphan.

There are only three Queens (who are all married to Fenris at once), and MC is the youngest of eleven children, many of whom are mentioned throughout the game.